There's a big difference between Priests and Shamans
Especially or Chain Heal vs Prayer of Healing (and this does the big difference in Healing Output)
While Chain Heal is jumping "intelligent" between all players in a raid, Prayer of Healing only heals the Targets subgroup (this is a problem every CC-Dev has to work out to get the best group to be used on), so the Healing is much higher for Chain Heal in most cases than for Prayer of Healing
Your comparisons are right that every class (or most classes) have nearly the same abilities, but for shamans are the mechanics much better
Rain goes on raid (all members staying in the rain will be healed), Chain Heal goes on raid (The cast decides by hisself where to jump (most injured )), where Priests has to select the right group
in worst case that means u have sanctuary up and heal your party for a low value + PoH up heal max 5 members (in most cases u get only 2 or 3, and maybe some heal goes to overhealing), Chain Heal goes to a target I choose (or the CC) and jumps to the most injured (nearly no overheal)
I know we could discuss this many many many Hours, the pros and cons bout healing classes

U r right with your standing, and i'm too
For the private developments: these are (atm) only ideas from me how i would like to have a healing CC (and only healing (incl. CDs, dispelling,maybe facing), no movement, no support for BG planned cause i do know nothing bout pvping) for raiding (nhc and hc) or dungeons.
Most of my problems are the usability, fast switching values for different bosses, fast ignoring members to be healed and so on, without restarting.
Ultimate was a good teacher to me, and has had much of the things i would like to see in this CC but it has had over (and this is really very much) 600 possible Settings per class (Paladin, HPriest, DPriest) which i could break down to only (around) 40 per class.
And this is still very much for setting it up, and i've not found an easy way to adjust them fast, and add new settings easily (for Singular as example u need one line for a new setting, Ultimate u needed 20 lines of code for a new settings -> Singular bad design for fast changing values, Ultimate good design... u need something between

)
So ... i'm still thinking bout such a CC and the more i think bout the more i want this and that ... and the Core is growing (atm only in my head) more and more (and this is the wrong way of starting development, u'll need a fix state u wanna reach before u begin to develop...i need a good concept paper ... which i'm working on atm)